To both masturbate and defecate simultaneously, usually on a lavatory or into a porcelain container.
Many begin to defecate before masturbating, so as to start an uninterrupted fecal flow. Also, it is not a requirement to ejaculate and finish defecating at the same time, but dedicated "shankers" will attempt the 'Snow on the Mountain' technique (wherein the discharge lands on the fecal matter.)
It is not necessary to be nude, but serial "shankers" will usually derobe before attempting a long shank.
Guy 1: "Jeez, you were ages man. What took you so long? Big shit?"
verb. typically thought of as a way to hurt someone in prison with a handmade knife. However, it has a 2nd meaning. Shank can also mean to draw a penis. The drawing can be as elaborate or as simple as one would like it to be. But the idea is that the penis drawing is a surprise, unbeknownst to the receiver.